February 19, 2011

mtDNA discontinuity in the Nile Valley

From the paper:

The distribution of subsets of haplogroups U6 and M1 also suggests the presence of a discontinuity between Libya and Egypt, separating western North Africa from eastern North Africa. Even if both haplogroups are thought to have been carried by a back-to-Africa migration from the Near East, significant increased U6 frequencies have been detected in the West compared to the East. The network of all U6 sequences found in the database presents two nodes with star-like shape, U6a* and U6a1. In a similar way, M1a1 is the node with starlike topology in haplogroup M1, and the node where most of the eastern sequences are found. Time estimates of these nodes are 13.5 +/- 3.7, 13.0 +/- 5.7, and 13.1 +/- 7.0 kya for haplogroups U6a*, U6a1, and M1a1 respectively. The most plausible explanation of the frequency distribution of M1a, U6, and M1b1 lineages, their coalescence age estimates, and the star-like shape would be an early split in the back to Africa migration followed by a period of stability and a period of expansion. The split would have produced two different migration waves, one westward, represented by U6 and possibly M1b1 in lower frequencies, and the other southward, represented by M1a.

Am J Phys Anthropol DOI: 10.1002/ajpa.21472

Mitochondrial DNA structure in North Africa reveals a genetic discontinuity in the Nile Valley

Karima Fadhlaoui-Zid et al.

Human population movements in North Africa have been mostly restricted to an east-west direction due to the geographical barriers imposed by the Sahara Desert and the Mediterranean Sea. Although these barriers have not completely impeded human migrations, genetic studies have shown that an east-west genetic gradient exists. However, the lack of genetic information of certain geographical areas and the focus of some studies in parts of the North African landscape have limited the global view of the genetic pool of North African populations. To provide a global view of the North African genetic landscape and population structure, we have analyzed ∼2,300 North African mitochondrial DNA lineages (including 269 new sequences from Libya, in the first mtDNA study of the general Libyan population). Our results show a clinal distribution of certain haplogroups, some of them more frequent in Western (H, HV0, L1b, L3b, U6) or Eastern populations (L0a, R0a, N1b, I, J) that might be the result of human migrations from the Middle East, sub-Saharan Africa, and Europe. Despite this clinal pattern, a genetic discontinuity is found in the Libyan/Egyptian border, suggesting a differential gene flow in the Nile River Valley. Finally, frequency of the post-LGM subclades H1 and H3 is predominant in Libya within the H sequences, highlighting the magnitude of the LGM expansion in North Africa.

The dividing line seems to be the Bedouin/Berber line outside the Nile Valley, with the Nile Valley itself having more affinity to the Bedoin than the Berber side of the line. Perhaps they each were good fits to such similar niches that they impeded expansion of the other beyond their territory.

Looking at the linguistic divides in Egypt, it also seems likely that what is called a "Libyan/Egyptian border" discontinuity really manifests at the edge of the Nile Delta rather than the true political border, as that is where the dialect of Arabic spoken in Libya and the dialect of Arabic spoken in Lower Egypt split. The Siwi Berber speakers of Western Egypt would presumably be on the Western side of the genetic divide as well.

The convergence dates and distributions seem a decent fit to the domestication of sheep and the earliest intentional cultivation of pre-domesticated plants in the epi-paleolithic. Thus, one can productively think of U6 and M1 as sheep herder lineages. It would be interesting to look at mtDNA convergence dates and haplotype mixes of sheep in the region to look for parallel patterns - I know some of that work has been done, but I am not very familiar with it.

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